Sven,
You've probably thought about this, but for Shearwater computers, an interim step might be to be able to import from the Shearwater Cloud app or maybe however it is that it saves into the cloud. I don't know how protective they are of the info/file formats, but from what I understand, they are open source friendly in general.

Smartphone apps are usually isolated from each other and cannot easily access the data from other apps. I have an import from DiveMate on Android, so if someone really wants to download the dives directly, they can do this via DiveMate and import into the Diving Log Android app.

One the PC, it depends on the exact Perdix version. If it is an Perdix AI with Bluetooth LE, it cannot be downloaded with Diving Log on the PC. If it is an older Perdix with classic Bluetooth, virtual COM ports are created by Windows, which can be used by Diving Log. It is planned to support also Bluetooth LE on the desktop, but this is also not so easy.